Periodic Reporting for period 4 - CMBSPEC (Next Steps in Cosmology with CMB Spectral Distortions)
Période du rapport: 2022-03-01 au 2023-08-31
The cosmic microwave background (CMB) has been an immense source of information about the Universe we live in. Not only has its detection in 1964 forged one of the main pillars of Big Bang Cosmology, but it also stimulated the first theoretical works on the CMB anisotropies in the early 1970's, which then defined the activities of the CMB community for the last few decades. The CMB, to leading order, is a uniform radiation field coming from the early phases of the expanding Universe that today is visible at microwave frequencies. Fluctuations in the density of the matter imprint tiny temperature and polarization anisotropies, which have been extensively studied, most recently using ESA's Planck Satellite. These observations clearly cemented the standard LCDM concordance model, which successfully describes cosmological observations over a vast range of scales.
However, another invaluable piece of information is provided by the energy distribution of the CMB photons, the so-called CMB spectrum. With COBE/FIRAS it was shown that the CMB spectrum is extremely close to that of a perfect blackbody at a temperature of 2.725 K, an observation that was awarded the Nobel Prize in Physics in 2006. This provided additional very strong support of the Big Bang picture, which naturally starts in a hot and dense phase that allows blackbody radiation to form. Importantly, even within LCDM we expect tiny departures of the CMB spectrum from that of a blackbody. These signals are commonly referred to as CMB spectral distortions and teach us about the thermal history of the Universe.
The main objective of CMBSPEC is to study CMB spectral distortion in the early Universe. Since COBE/FIRAS in the early 1990's, no serious observational attempt was made to measure the CMB energy spectrum more precisely. However, technologically we have now reached a stage that should allow us to detect some of the predicted LCDM distortion signals. In addition, our theoretical understanding of how CMB spectral distortions form has evolved significantly since the earliest predictions in the 1970's. The CMBSPEC team is thus working hard to facilitate new observations of the CMB spectrum by answering important questions about
1) the formation of distortions in the early Universe
2) how to analyze and interpret future distortion measurements
3) which experimental concepts will provide the most stringent limits
4) how can we overcome and deal with spectral distortions foregrounds
CMB spectral distortions are extremely small and heavily obscured by galactic and extra-galactic foregrounds. We have extensive experience with CMB foregrounds from observations of the CMB anisotropies, and several aspects directly translate to CMB spectral distortions. However, the dynamic range encountered in signal to foreground is huge and innovative methods have to be developed to extract the distortions. In addition, systematic effects are very different given that for CMB spectral distortion measurements an absolute calibration or at least pristine channel-intercalibration is needed. The work of the CMBSPEC team provides important input for the experimental design and optimization of CMB spectrometers and how these can be combined with planned CMB imagers to overcome the challenges.
Another important question is how spectral distortions form and evolve in the early Universe. The CMBSPEC team has shown that the distortion signals directly depend on when in cosmic history the distortion was imprinted. In addition, there are differences in distortion signals that are created by heating and photon injection. The details of how to link the shape and amplitude of the distortion to specific processes are crucial for the interpretation of future distortion data and my team is working intensively on understanding how to model and analyze distortions from various mechanism. One of the big hopes and drivers ultimately is to learn about primordial inflation and dark matter by studying CMB spectral distortion, thus targeting highly fundamental questions about the Universe we live in.
Through CMBSPEC, the team has achieved several important milestones. CMB spectral distortions are now widely recognized as an important target for the future. BISOU, a french-led balloon-borne CMB spectrometer is set to start Phase A in 2024, promising the first new measurements of the CMB spectrum since COBE/FIRAS by the end of this decade. A number of ground-based spectrometers (COSMO and TMS) are also progressing towards measurements of distortions and the long-term prospects for a CMB space mission are extremely promising. The team has furthermore opened a novel direction in CMB distortion science, for the first time treating the full spectro-spatial thermalization problem in the perturbed universe, defining new spectral distortion targets for Litebird and CMB-S4.
However, another invaluable piece of information is provided by the energy distribution of the CMB photons, the so-called CMB spectrum. With COBE/FIRAS it was shown that the CMB spectrum is extremely close to that of a perfect blackbody at a temperature of 2.725 K, an observation that was awarded the Nobel Prize in Physics in 2006. This provided additional very strong support of the Big Bang picture, which naturally starts in a hot and dense phase that allows blackbody radiation to form. Importantly, even within LCDM we expect tiny departures of the CMB spectrum from that of a blackbody. These signals are commonly referred to as CMB spectral distortions and teach us about the thermal history of the Universe.
The main objective of CMBSPEC is to study CMB spectral distortion in the early Universe. Since COBE/FIRAS in the early 1990's, no serious observational attempt was made to measure the CMB energy spectrum more precisely. However, technologically we have now reached a stage that should allow us to detect some of the predicted LCDM distortion signals. In addition, our theoretical understanding of how CMB spectral distortions form has evolved significantly since the earliest predictions in the 1970's. The CMBSPEC team is thus working hard to facilitate new observations of the CMB spectrum by answering important questions about
1) the formation of distortions in the early Universe
2) how to analyze and interpret future distortion measurements
3) which experimental concepts will provide the most stringent limits
4) how can we overcome and deal with spectral distortions foregrounds
CMB spectral distortions are extremely small and heavily obscured by galactic and extra-galactic foregrounds. We have extensive experience with CMB foregrounds from observations of the CMB anisotropies, and several aspects directly translate to CMB spectral distortions. However, the dynamic range encountered in signal to foreground is huge and innovative methods have to be developed to extract the distortions. In addition, systematic effects are very different given that for CMB spectral distortion measurements an absolute calibration or at least pristine channel-intercalibration is needed. The work of the CMBSPEC team provides important input for the experimental design and optimization of CMB spectrometers and how these can be combined with planned CMB imagers to overcome the challenges.
Another important question is how spectral distortions form and evolve in the early Universe. The CMBSPEC team has shown that the distortion signals directly depend on when in cosmic history the distortion was imprinted. In addition, there are differences in distortion signals that are created by heating and photon injection. The details of how to link the shape and amplitude of the distortion to specific processes are crucial for the interpretation of future distortion data and my team is working intensively on understanding how to model and analyze distortions from various mechanism. One of the big hopes and drivers ultimately is to learn about primordial inflation and dark matter by studying CMB spectral distortion, thus targeting highly fundamental questions about the Universe we live in.
Through CMBSPEC, the team has achieved several important milestones. CMB spectral distortions are now widely recognized as an important target for the future. BISOU, a french-led balloon-borne CMB spectrometer is set to start Phase A in 2024, promising the first new measurements of the CMB spectrum since COBE/FIRAS by the end of this decade. A number of ground-based spectrometers (COSMO and TMS) are also progressing towards measurements of distortions and the long-term prospects for a CMB space mission are extremely promising. The team has furthermore opened a novel direction in CMB distortion science, for the first time treating the full spectro-spatial thermalization problem in the perturbed universe, defining new spectral distortion targets for Litebird and CMB-S4.
Since the start of the ERC grant, my team has been at the forefront of motivating and leading the community toward future measurements of CMB spectral distortions. We addressed the challenges by considering all the above questions more or less simultaneously. This was only possible through funding by the ERC that allowed the PI to put together a diverse team of researchers in a very short time.
Significant progress was made on the theoretical understanding of CMB spectral distortions with advanced extensions of the existing codes CosmoTherm and CosmoSpec. Most significantly, we are now able to include the signals from energy release caused by gravitational waves and also model the evolution of anisotropic distortions. Additional distortions caused by the scattering of the cosmic radio and infrared background have been newly worked out by the team, giving further big motivation to push the experimental frontier.
During the CMBSPEC project, the team has contributed to a number ESA and NASA mission proposals. CMB spectral distortions were furthermore recognized as one of the important future probes of early-universe physics in the ESA Voyage 2050 space program and three sub-orbital CMB spectrometers (TMS, COSMO and BISOU) are now entering advanced stages. Again the outputs from the CMBSPEC team were crucial in these developments and former team members are now playing important roles in these activities.
Significant progress was made on the theoretical understanding of CMB spectral distortions with advanced extensions of the existing codes CosmoTherm and CosmoSpec. Most significantly, we are now able to include the signals from energy release caused by gravitational waves and also model the evolution of anisotropic distortions. Additional distortions caused by the scattering of the cosmic radio and infrared background have been newly worked out by the team, giving further big motivation to push the experimental frontier.
During the CMBSPEC project, the team has contributed to a number ESA and NASA mission proposals. CMB spectral distortions were furthermore recognized as one of the important future probes of early-universe physics in the ESA Voyage 2050 space program and three sub-orbital CMB spectrometers (TMS, COSMO and BISOU) are now entering advanced stages. Again the outputs from the CMBSPEC team were crucial in these developments and former team members are now playing important roles in these activities.
The team developed new methods for modeling some of the key processes involved in the evolution of CMB spectral distortions in the early Universe. These software packages provide the basis for many additional computations of distortions signals, but beyond that will find applications in a wide range of astrophysical problems. This work has essentially eliminated the necessity for simplifying approximations in the description of Compton scattering, Bremsstrahlung and double Compton emission events.
The extensive progress of the team on component separation methods that combine statistical moment-expansion methods with more traditional methods known in the context of CMB anisotropies have several applications beyond CMB spectral distortions (e.g. B-modes). The CMBSPEC team also managed to formulate and solve the problem of spectro-spatial distortion evolution. This achievement was not anticipated at the beginning of the grant and opens novel opportunities for CMB imagers like Planck, Litebird, CMB-S4 and SO to constrain CMB spectral distortion physics, with a wide range of new projects seeded by this theoretical breakthrough.
The extensive progress of the team on component separation methods that combine statistical moment-expansion methods with more traditional methods known in the context of CMB anisotropies have several applications beyond CMB spectral distortions (e.g. B-modes). The CMBSPEC team also managed to formulate and solve the problem of spectro-spatial distortion evolution. This achievement was not anticipated at the beginning of the grant and opens novel opportunities for CMB imagers like Planck, Litebird, CMB-S4 and SO to constrain CMB spectral distortion physics, with a wide range of new projects seeded by this theoretical breakthrough.